The present invention relates to an image intensifier tube target and also to image intensifier tubes having a video output provided with such a target.
The following description essentially relates to radiological image intensifier tubes, known as R.I.I. However, it is obvious that the invention also applies to luminous image intensifier tubes and to scintiscanning image intensifier tubes (.gamma. radiation).
In the case of R.I.I. tubes, it is desirable to have variable gain targets, whose gain, i.e. the number of photons emitted for each electron received by the target, can be multiplied by a factor of about 100. Thus, if desired, the R.I.I. can operate in radiography or in fluoroscopy.
In radiography the R.I.I. video output signal makes it possible to display on a television screen the information contained in the X-ray beam reaching the R.I.I. and the television picture is recorded on film or photograph. A high X-ray dose must be transmitted during the short exposure time to obtain a good signal-to-noise ratio. To prevent saturation, it is necessary to have a low gain target.
In fluoroscopy, the television screen is directly observed and a low X-ray dose is transmitted during the relatively long observation time. It is then necessary to have a high gain target to obtain a good image. U.S. Pat. No. 4,029,965, discloses an R.I.I. target with a video output and which has a variable gain. It is in fact a silicon target, one of whose faces is covered with a luminescent coating, itself covered with a metallic barrier layer, as an entering layer.
The electron beam from the R.I.I. cathode reaches the metallic barrier layer, which slows it down and only permits the passage of the higher energy electrons. In the luminescent coating, these electrons bring about the formation of photons, which produce charge carriers in the silicon of the target. These charge carriers discharge reverse-polarized diodes located on the other face of the target. Finally, the charge distribution on the other face of the target is scanned by the electron beam of a camera tube, which supplies the video signal.
The gain variation of the target is obtained by varying the accelerating voltage of the R.I.I. beam and by using the non-linear relationship which exists for metallic barrier layers between the penetration of the electrons into the barrier layer and the accelerating voltage of the electron beam.
However, this prior art variable gain target has the disadvantages that the resolution of the R.I.I. is reduced through the use of two layers covering the silicon target, namely the metallic barrier layer and the luminescent coating, whilst the presence of a metallic barrier layer introduces noise and leads to defects in the image obtained, as is indicated in U.S. Pat. No. 4,029,965.